Recently, it has been shown how thin film composite hollow fiber membranes can be prepared for forward osmosis (Sukitpaneenit & Chung 2012) through interfacial polymerization creating a polyamide thin layer on the inner surface of PES hollow fiber support membranes where a 2 wt % of meta-phenylene diamine (MPD) aqueous solution containing 0.5 wt % of triethylamine and 0.1 wt % of sodium dodecyl sulfate is introduced through pumping from a bottom inlet into a vertically positioned HF module followed by air drying and then introduction of a 0.15 wt % of trimesoyl chloride (TMC) in hexane solution to form the polyamide thin film and finally purging with air and curing at 65° C., rinsing with deionized water and storing in deionized water. Furthermore, Peinemann et al. describes a method of preparing a similar TFC layer on the interior surface of a hollow fiber, cf. US published patent application No. 2007/0199892. Moreover, Zhong et al. (2013) describe the development of TFC forward osmosis hollow fiber membranes using direct sulfonated polyphenylenesulfone as membrane substrate. However, for a wide range of separation applications where hollow fiber modules are employed it is of importance to be able to separate or filter out organic solutes of relatively low molecular weight during the filtration process. For example, in haemodialysis where HF modules are widely used, a uremic toxin and organic degradation product, such as indoxyl sulfate (Indoxyl sulfate potassium salt, CAS Number 2642-37-7, molecular weight 251.30) poses a particularly difficult problem. Indoxyl sulfate is accumulated in the serum of chronic kidney disease patients. A part of the dietary protein-derived tryptophan is metabolized into indole by tryptophanase in intestinal bacteria. Indole is absorbed into the blood from the intestine, and is metabolized to indoxyl sulfate in the liver. Indoxyl sulfate is normally excreted into urine. In haemodialysis patients, however, an inadequate renal clearance of indoxyl sulfate leads to its elevated serum levels, cf. Niwa T. (2010). Niwa et al. (1997) have advanced the hypothesis that accumulation of indoxyl sulfate accelerates glomerular sclerosis and progression of kidney disease. Administration of an oral adsorbent lowers indoxyl sulfate levels in undialyzed uraemic patients, cf Niwa et al. (1997). Current dialysis methods, i.e. haemodialysis and peritoneal dialysis, the latter being characterised by continuous ultrafiltration and solute removal, do not sufficiently remove some low molecular weight degradation products from serum, such as indoxyl sulfate and p-cresol (4-methylphenol, CAS No. 106-44-5, molecular weight 108.13). In addition, small water-soluble molecules, such as urea, uric acid and creatinine, and peptides/proteins, such as β2-microglobulin should preferably be removed during dialysis. A direct association between p-cresol, mainly reflecting p-cresyl sulfate, and overall mortality and cardiovascular disease in end-stage renal disease and in chronic kidney disease has been found. Likewise, direct associations between indoxyl sulfate and overall mortality and cardiovascular disease has been reported. In continuous hemofiltration therapy loss of physiological (vital) proteins should be minimized and removal of low (<500 Da) and middle molecular weight (from about 500 to about 40 kDa) uremic toxins and peptides should be optimized, cf. Wenhao Xie (2011).
Aoike (2011) mentions the most important features or quality parameters for high performance membranes for use in hemopurification therapies, i.e., high water permeability, capability to remove a wide range of uremic toxins and other characteristic features. However, Aoike also points out that large pore size of existing high performance membranes (HPM) will likely allow blood to be contaminated by the dialysis fluid, because HPMs, such as polyacrylonitrile (PAN) membranes, having a large pore size allow untoward rapid diffusion of dialyzed endotoxin fragments back into the blood compartment.
Accordingly, it remains a problem in the art in providing hollow fiber modules that are able to separate low molecular weight compounds, enabling their removal from liquids in processes such as haemodialysis and in which the hollow fibre module combine a high water permeability with smaller pore sizes.